Back to NeuroanatomyDura Mater
The dura mater or pacymeninx ( thick membrane ) forms a tough and fibrous outer covering divided into two portions. The external layer of the dura mater, derived from the internal layer of cranial periosteum, is adherent to the inner surface of the skull, and continuous with the ectocranial periosteum through the foramina of the skull. The internal layer, is the true meningeal dura mater. It is called cranial dura mater and adheres in most places to the external layer. Separations of the two layers create the dural sinuses, the dural refections, and the trigeminal cave ( of Meckel ). The internal layer continues through the magnum foramen to encloses the spinal cord ending in the second segment of the sacrum.
Dural Reflections
The internal layer forms the five dural reflections, namely, the falx cerebri, the falx cerebelli, the tentorium cerebelli, the diaphragma sellae and the tentorium bulbus olfactorium.
The tentorium cerebelli is the most important reflection. It is situated between the cerebellum and the occipital and posterior portions of the temporal lobes of the cerebrum, divinding the cranial cavityinto supratentorial and subtentorial compartments.
The falx cerebri is a midline, sickle-shaped reflection. It divides the supratentorial compartment into a right and left side. It attaches anteriorly to the crista galliand frontal crest, and posteriorly the lower border joins the tentorium cerebelli.
The falx cerebelli is a small dural reflection, between the posterosuperior portions of the cerebellar hemispheres.
The diaphragma sellae is a horizontal, circular fold which cover the sella turcica or pituitary fossa of the sphenoid bone, separating the pituitary gland from the hypothalamus and opitc chiasm. It has an central aperture for passage of the infundibular stem of hypophysis and portal vein.
The tentorium bulbus olfactorium is a small reflection in the anterior portion of the olfactory fossa which floor is the cribiform plate of the ethmoid bone. The reflection pushes the olfactory bulb over the bone.
Dural Venous Sinuses
The separations of the two layers of the dura mater create the dural venous sinuses. The venous sinuses receive the blood from the veins draining the brain, and this blood flows from the venous sinuses to the internal jugular veins. The walls of the sinuses are line by endothelium.
The Sinuses of the Calvaria
The superior sagittal sinus lies along the superior border of the falx cerebri. It begins at the foramen cecum, receving tributaries from the nasal veins, continuing posteriorly in the sagittal suture, terminating at the confluence of the sinuses ( torcular Herophili ).
The inferior sagittal sinus runs backward along the lower or free margin of the falx cerebri to opens into the straight sinus.
The straight sinus runs in the attachment of the falx cerebri to the tentorium cerebelli. It is formed by the union of the inferior sagittal sinus with the great cerebral vein of galen.
The transverse sinuses ( left and right ) receive the blood from the confluence of the sinuses. The right tranverse sinus is usually the continuation of the superior sagittal sinus, while the left tansverse sinus is the prolongation of the straight sinus. The transvers sinuses lie along the attached margin of the tentorium cerebelli in the sulcus of transverse sinus. When they reach the petrous portion of the temporal bone, each of them continues as the sigmoid sinus.
The sigmoid sinuses follow a S-shapped course on the mastoid portion of the petrous part of the temporal bone. They end at the jugular foramina to continue as the internal jugular veins.
The Sinuses of Base of Skull
The occipital sinus lies in the roof of the falx cerebelli, emptying into the confluence of the sinuses.
The sphenoparietal sinus ( of Breschet ) runs under the lesser of the sphenoid bone draining into the cavernous sinus.
The cavernous sinus is situated laterally to the body of the sphenoid bone. It is a plexus formed by some separeted veins. The cavernous sinuses receive the superior and inferior ophthalmic veins, the central vein of retina, the middle and inferior cerebral veins, and the sphenoparietal sinuses. The internal carotid artery and the abducens nerve ( VI ) run medially within sheaths of endothelium. The ophthalmic ( V.1 ), and maxillary ( V.2 ) branches of the trigeminal nerve, the oculomotor ( III ) and the trochlear ( IV ) nerves pass forward more laterally along the wall of the cavernous sinuses. They drain into the inferior and superior petrosal sinuses.
The intercavernous sinus ( circular sinus ) is situated in the diaphragma sellae, connecting the cavernous sinuses. It forms a venous collar around the infundibular stem of the hypophysis.
The basilar plexus is located over the basilar portion of the occipital bone. It has two portions ( superior and inferior ) draining into the inferior petrosal sinuses. The superior portion receives the blood from the cavernous sinuses, and the inferior portion receives the blood from the anterior vertebral plexuses.
The superior petrosal sinus provides a communication between the cavernous sinus and the terminal portion of the transverse sinus. It runs along the superior margin of the petrous part of the temporal bone in the attachment of the tentorium cerebelli.
The inferior petrosal sinus drains into the internal jugular veins the blood from the cavernous sinus and from the anterior vertevral plexuses. It runs in the petrooccipital suture.
The tentorium sinus within the cerebellum tentorium receives the blood from the superior veins of the cerebellar hemisphere and drains into the transverse sinus.
The marginal sinus receives the blood from the posterior internal vertebral venous plexus. The two marginal sinuses join to form the occipital sinus.
The Trigeminal Cave ( Meckel’s Cave )
In the middle cranial fossa, lateral to the tip of the petrous portion of the temporal bone is the trigeminal impression which contains the trigeminal cave, a space formed by the separation of the two layers of the dura mater. The trigeminal ganglion ( of Gasser ), the trigeminal nerve and its three branches are located inside the trigeminal cave.
Internal to the dura mater is a thin meninge called arachnoid mater. It is an avascular membrane separeted from the dura mater by a potential subdural space that contains just the sufficient fluid to the lubrification of the two membranes.
The arachnoid space is a relatively large space that contains the cerebrospinal fluid. That membrane is called arachnoid because the cobweblike trabeculae crossing the subarachnoid to become continuous with the pia mater. The ventricular system communicates with the subarachnoid space through the median aperture ( of Magendie ) and through the lateral apertures ( of Luschka ) situated in the fourth ventricle.
Pia mater is a delicate layer constituting with the arachnoid the leptomeninges ( slender membranes ). It is a highly vascular layer, closely adherent to the brain and spinal cord, following all their contours.
Subarachnoid Cisterns
The arachnoid rests on the dura mater and they just accompanish grossly the brain surface. The pia mater, however, adheres closely to the irregular contours of the brain and spinal cord, and the width of the subarachnoid space becomes deeper in the region of major sulci, containing more amounts of cerebrospinal fluid, forming the subarachnoid cisterns.
The cerebrospinal fluid ( CSF ) from the ventricles, escapes into the subarachnoid space through the median aperture ( of magendie ) into the cerebellomedullary cistern ( cisterna magna ) and through the lateral apertures ( of Luschka ) into the pontine cistern.
The CSF then circulates through the others subarachnoid cisterns at the base of the brain. These are: (1) The cerebellomedullary cistern which occupies the interval between the cerebellum and the medulla oblongata; (2) The pontine cistern which lies in the anteroinferior portion of the pons; (3) The interpeduncular cistern, which is located on the anterior surface of the midbrain, between the cerebal peduncles; (4) The chiasmatic cistern surrounds the optic chiasm and is continuous with the cistern of terminal lamina; (5) The cistern of the corpus callosum ( supracallosal cistern ) is located above this commissure receiving CSF from the cistern of terminal lamina; (6) The cistern of the lateral cerebral foss ( cistern of the lateral sulcus ) corresponds with the lateral ( sylvian ) sulcus; (7) The superior cistern ( great cerebral vein cistern ) is located behind to the midbrain below to the splenium of corpus callosum; (8) The cisterna ambiens lies on the sides of the midbrain; (9) The trigeminal nerve cirtern surrounds that nerve; (10) The lumbar cistern surrounds the spinal cord, extending from the second lumbar vertebra to the second segment of the sacrum and contains the cauda equina.
Cerebrospinal Fluid ( CSF )
The CSF is produced by the choroid plexuses of the lateral, third and fourth ventricles. The choroid plexuses are formed by an invagintion of the vascular pia mater ( the tela choroidea )
The CSF produced in the lateral ventricles pass througth the interventricular foramina ( of Monro ) to the third ventricle and then through the cerebral aqueduct ( of Sylvius ) into the fourth ventricle. The four ventricles possess choroid plexuses augmenting the quantity of CSF. The CSF reaches the subarachnoid space and its cisterns through the median aperture ( of Magendie ) and Through the lateral apertures ( of Luschka ) of the fourth ventricle. From the cisterns, most of the CSF flows upward over the medial and lateral surfaces of the cerebral hemispheres toward the superior sagittal sinus, but smaller amounts of CSF flows downward around the spinal subarachnoid space.
Pacchioni described granulations over the meninges in 1721. That structures, namely arachnoid vili, are the main site of absorption of the CSF into venous blood of the dural venous sinuses. The arachnoid villi are more abundant in thesuperior sagittal sinus and consist of an extension of the subarachnoid space into the dura mater. The separation between the subarachnoid space and the venous blood is made by a thin cellular layer, derived from the epithelium of the arachnoid and the endothelium of the sinus. The arachnoid villi are notcovered by the dura mater. In the adult and the old people the villi become bigger and are called pacchionian bodies or arachnoid granulations. Frequently they become calcified and let impressions in the calvaria.
The cerebrospinal fluid has several functions. It mantains a constant external environment for neurons and glia. The CSF removes the brain metabolites. The CSF forms a mechanical cushion between the central nervous system and the bony surrounding. Any pressure exerted over a point of this fluid cushion is equaly divided for all fluid, weakening the impact over the central nervous system. The brain floats on the CSF, what reduces the brain weight to less than 50g. The brain weights more than 1.200g. in the air. The CSF serves as a lymphatic system. The cerebral blood flow and the pulmonary ventilation are affected by the pH of CSF
Epidural Hematoma
The middle meningeal artery, a branch of the maxillary artery, passes through the foramen spinosum in the floor of the middle cranial fossa, dividing in anterior and posterior branches which produce grooves on the temporal and parietal bones. After a blow on the side of the head that arteries may rupture causing an epidural hemorrhage. About 15% of the epidural hematomas arise from bleeding of veins or venous sinuses.
The hemorrhage occurs in a portion which the external layer of dura mater is easily separable from the bones, the separable zone of Gérard - Marchant. After a brief period of unconsciousness the patient recover the consciousness, initiating the lucid interval during which there are minimal symptons or signs, but the hematoma continues to enlarge compressing the hemisphere. There is compression of the oculomotor nerve, dilatation of the pupil, abduction of the eye and palpebral ptosis, that signis are ipsilateral to the hematoma. Compression of the cerebral peduncle causes contralateral hemiparesis, progressing to decerebrate posturing. Coma, fixed and dilated pupil (s), and decerebration are the triad indicating transtentorial herniation.
Epidural hematomas are curable lesions, but the mortality is high because they are usually undiagnosed since several times the patient is in the lucid interval when arrives at the hospital.
Subdural Hematoma
The larger cerebral veins cross through the arachnoid layer and the subdural space to drain into the venous sinuses. As a result of a blow to the head, the veins may rupture. The subdural hematomas can be classified as acute, subacute and chronic subdural hematomas.
The production of CSF varies with the intracranial pressure. When the pressure raises the production of the CSF diminishes and pressure diminishes too. It is responsible for the usually chronic symptoms.
Subarachnoid Hemorraghe ( SAH )
The SAH can occur from traumatic and nontraumatic causes and may be rapidly fatal. The most commom cause for nontraumatic hemorraghe is aneurysm rupture. The blood accumulates in the subarachnoid space and can be visualized by the lumbar puncture. There is usually a history of sudden severe headache with onset correlated to strenous activity.
Hydrocephalus
Hydrocephalus is an increase in the amount of CSF within the ventricular system. There are two types of hydrocephalus, the communicating and noncommunicating hydrocephalus. In the former, the ventricular system continues to communicate with the subarachnoid space through the apertures of the fourth ventricle. In the noncommunicating or obstructive there isn’t continuity of the ventricular system and the subarachnoid space, what is responsible for the accumulation of the CSF.
Meningioma
From the arachnoid mater can arise a relatively benign tumor called meningioma, which is more commom between the the fourth and sixth decades of life and affect women more commonly than men ( 3:2 ).
Infections
The infection of the meninges is a commom disease worldwide, called meningites. The bacterial meningitis is the most commom type and may be defined as an inflamatory response to the bacterial infection of the pia-arachnoid and the CSF.
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